Add [snafu(implicit)]

compatibility-tests/compile-fail/tests/ui/attribute-misuse.rs

@@ -7,6 +7,7 @@ mod enum_misuse {
     #[snafu(source(true))]
     #[snafu(backtrace)]
     #[snafu(context)]
+    #[snafu(implicit)]
     enum EnumError {
         AVariant,
     }
@@ -22,6 +23,7 @@ mod variant_misuse {
         #[snafu(source)]
         #[snafu(backtrace)]
         #[snafu(crate_root(XXXX))]
+        #[snafu(implicit)]
         AVariant,
     }
 }
@@ -56,6 +58,7 @@ mod struct_misuse {
     #[snafu(source(true))]
     #[snafu(backtrace)]
     #[snafu(context)]
+    #[snafu(implicit)]
     struct StructError(Box<UsableError>);
 }
 

compatibility-tests/compile-fail/tests/ui/attribute-misuse.stderr

@@ -28,86 +28,104 @@ error: `context` attribute is only valid on enum variants or structs with named
 9 |     #[snafu(context)]
   |             ^^^^^^^
 
+error: `implicit` attribute is only valid on enum variant or struct fields with a name, not on an enum
+  --> $DIR/attribute-misuse.rs:10:13
+   |
+10 |     #[snafu(implicit)]
+   |             ^^^^^^^^
+
 error: `source` attribute is only valid on enum variant or struct fields with a name, not on an enum variant
-  --> $DIR/attribute-misuse.rs:21:46
+  --> $DIR/attribute-misuse.rs:22:46
    |
-21 |         #[snafu(display("an error variant"), source(from(XXXX, Box::new)))]
+22 |         #[snafu(display("an error variant"), source(from(XXXX, Box::new)))]
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `source` attribute is only valid on enum variant or struct fields with a name, not on an enum variant
-  --> $DIR/attribute-misuse.rs:22:17
+  --> $DIR/attribute-misuse.rs:23:17
    |
-22 |         #[snafu(source)]
+23 |         #[snafu(source)]
    |                 ^^^^^^
 
 error: `backtrace` attribute is only valid on enum variant or struct fields with a name, not on an enum variant
-  --> $DIR/attribute-misuse.rs:23:17
+  --> $DIR/attribute-misuse.rs:24:17
    |
-23 |         #[snafu(backtrace)]
+24 |         #[snafu(backtrace)]
    |                 ^^^^^^^^^
 
 error: `crate_root` attribute is only valid on an enum or a struct, not on an enum variant
-  --> $DIR/attribute-misuse.rs:24:17
+  --> $DIR/attribute-misuse.rs:25:17
    |
-24 |         #[snafu(crate_root(XXXX))]
+25 |         #[snafu(crate_root(XXXX))]
    |                 ^^^^^^^^^^^^^^^^
 
+error: `implicit` attribute is only valid on enum variant or struct fields with a name, not on an enum variant
+  --> $DIR/attribute-misuse.rs:26:17
+   |
+26 |         #[snafu(implicit)]
+   |                 ^^^^^^^^
+
 error: `display` attribute is only valid on enum variants or structs with named fields, not on a field
-  --> $DIR/attribute-misuse.rs:35:21
+  --> $DIR/attribute-misuse.rs:37:21
    |
-35 |             #[snafu(display("display should not work here"))]
+37 |             #[snafu(display("display should not work here"))]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `visibility` attribute is only valid on an enum, enum variants, or a struct with named fields, not on a field
-  --> $DIR/attribute-misuse.rs:36:21
+  --> $DIR/attribute-misuse.rs:38:21
    |
-36 |             #[snafu(visibility(pub))]
+38 |             #[snafu(visibility(pub))]
    |                     ^^^^^^^^^^^^^^^
 
 error: Incompatible attributes [`source(false)`, `source(from)`] specified on a field
-  --> $DIR/attribute-misuse.rs:38:21
+  --> $DIR/attribute-misuse.rs:40:21
    |
-38 |             #[snafu(source(from(XXXX, Box::new)))]
+40 |             #[snafu(source(from(XXXX, Box::new)))]
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `context` attribute is only valid on enum variants or structs with named fields, not on a field
-  --> $DIR/attribute-misuse.rs:39:21
+  --> $DIR/attribute-misuse.rs:41:21
    |
-39 |             #[snafu(context)]
+41 |             #[snafu(context)]
    |                     ^^^^^^^
 
 error: `crate_root` attribute is only valid on an enum or a struct, not on a field
-  --> $DIR/attribute-misuse.rs:40:21
+  --> $DIR/attribute-misuse.rs:42:21
    |
-40 |             #[snafu(crate_root(XXXX))]
+42 |             #[snafu(crate_root(XXXX))]
    |                     ^^^^^^^^^^^^^^^^
 
 error: `display` attribute is only valid on enum variants or structs with named fields, not on a tuple struct
-  --> $DIR/attribute-misuse.rs:53:13
+  --> $DIR/attribute-misuse.rs:55:13
    |
-53 |     #[snafu(display("display should not work here"))]
+55 |     #[snafu(display("display should not work here"))]
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `visibility` attribute is only valid on an enum, enum variants, or a struct with named fields, not on a tuple struct
-  --> $DIR/attribute-misuse.rs:55:13
+  --> $DIR/attribute-misuse.rs:57:13
    |
-55 |     #[snafu(visibility(pub))]
+57 |     #[snafu(visibility(pub))]
    |             ^^^^^^^^^^^^^^^
 
 error: `source(bool)` attribute is only valid on enum variant or struct fields with a name, not on a tuple struct
-  --> $DIR/attribute-misuse.rs:56:13
+  --> $DIR/attribute-misuse.rs:58:13
    |
-56 |     #[snafu(source(true))]
+58 |     #[snafu(source(true))]
    |             ^^^^^^^^^^^^
 
 error: `backtrace` attribute is only valid on enum variant or struct fields with a name, not on a tuple struct
-  --> $DIR/attribute-misuse.rs:57:13
+  --> $DIR/attribute-misuse.rs:59:13
    |
-57 |     #[snafu(backtrace)]
+59 |     #[snafu(backtrace)]
    |             ^^^^^^^^^
 
 error: `context` attribute is only valid on enum variants or structs with named fields, not on a tuple struct
-  --> $DIR/attribute-misuse.rs:58:13
+  --> $DIR/attribute-misuse.rs:60:13
    |
-58 |     #[snafu(context)]
+60 |     #[snafu(context)]
    |             ^^^^^^^
+
+error: `implicit` attribute is only valid on enum variant or struct fields with a name, not on a tuple struct
+  --> $DIR/attribute-misuse.rs:61:13
+   |
+61 |     #[snafu(implicit)]
+   |             ^^^^^^^^

compatibility-tests/compile-fail/tests/ui/duplication-implicit-attributes.rs

@@ -0,0 +1,22 @@
+use snafu::prelude::*;
+
+#[derive(Debug)]
+struct ImplicitData;
+
+impl snafu::GenerateImplicitData for ImplicitData {
+    fn generate() -> Self {
+        Self
+    }
+}
+
+#[derive(Debug, Snafu)]
+enum EnumError {
+    AVariant {
+        // Second attribute should be marked as duplicate
+        #[snafu(implicit)]
+        #[snafu(implicit)]
+        my_data: ImplicitData,
+    },
+}
+
+fn main() {}

compatibility-tests/compile-fail/tests/ui/duplication-implicit-attributes.stderr

@@ -0,0 +1,5 @@
+error: Multiple `implicit` attributes are not supported on a field
+  --> $DIR/duplication-implicit-attributes.rs:17:17
+   |
+17 |         #[snafu(implicit)]
+   |                 ^^^^^^^^

compatibility-tests/compile-fail/tests/ui/structs/attribute-misuse.stderr

@@ -1,4 +1,4 @@
-error: expected one of: `backtrace`, `context`, `crate_root`, `display`, `source`, `visibility`, `whatever`
+error: expected one of: `backtrace`, `context`, `crate_root`, `display`, `implicit`, `source`, `visibility`, `whatever`
  --> $DIR/attribute-misuse.rs:5:13
   |
 5 |     #[snafu(unknown_attribute)]

snafu-derive/src/lib.rs

@@ -41,6 +41,7 @@ struct EnumInfo {
 struct FieldContainer {
     name: syn::Ident,
     backtrace_field: Option<Field>,
+    implicit_fields: Vec<Field>,
     selector_kind: ContextSelectorKind,
     display_format: Option<UserInput>,
     doc_comment: String,
@@ -500,6 +501,11 @@ const ATTR_BACKTRACE_FALSE: WrongField = WrongField {
     valid_field: "backtrace",
 };
 
+const ATTR_IMPLICIT: OnlyValidOn = OnlyValidOn {
+    attribute: "implicit",
+    valid_on: "enum variant or struct fields with a name",
+};
+
 const ATTR_VISIBILITY: OnlyValidOn = OnlyValidOn {
     attribute: "visibility",
     valid_on: "an enum, enum variants, or a struct with named fields",
@@ -554,6 +560,7 @@ fn parse_snafu_enum(
             }
             Att::CrateRoot(tokens, root) => crate_roots.add(root, tokens),
             Att::Backtrace(tokens, ..) => enum_errors.add(tokens, ATTR_BACKTRACE),
+            Att::Implicit(tokens, ..) => enum_errors.add(tokens, ATTR_IMPLICIT),
             Att::Context(tokens, ..) => enum_errors.add(tokens, ATTR_CONTEXT),
             Att::Whatever(tokens) => enum_errors.add(tokens, ATTR_WHATEVER),
             Att::DocComment(..) => { /* Just a regular doc comment. */ }
@@ -642,6 +649,7 @@ fn field_container(
             Att::Whatever(tokens) => whatevers.add((), tokens),
             Att::Source(tokens, ..) => outer_errors.add(tokens, ATTR_SOURCE),
             Att::Backtrace(tokens, ..) => outer_errors.add(tokens, ATTR_BACKTRACE),
+            Att::Implicit(tokens, ..) => outer_errors.add(tokens, ATTR_IMPLICIT),
             Att::CrateRoot(tokens, ..) => outer_errors.add(tokens, ATTR_CRATE_ROOT),
             Att::DocComment(_tts, doc_comment_line) => {
                 // We join all the doc comment attributes with a space,
@@ -665,6 +673,7 @@ fn field_container(
     let mut user_fields = Vec::new();
     let mut source_fields = AtMostOne::new("source", inner_error_location);
     let mut backtrace_fields = AtMostOne::new("backtrace", inner_error_location);
+    let mut implicit_fields = Vec::new();
 
     for syn_field in fields {
         let original = syn_field.clone();
@@ -689,6 +698,7 @@ fn field_container(
         // don't need any more data.
         let mut source_attrs = AtMostOne::new("source", ErrorLocation::OnField);
         let mut backtrace_attrs = AtMostOne::new("backtrace", ErrorLocation::OnField);
+        let mut implicit_attrs = AtMostOne::new("implicit", ErrorLocation::OnField);
 
         // Keep track of the negative markers so we can check for inconsistencies and
         // exclude fields even if they have the "source" or "backtrace" name.
@@ -740,6 +750,11 @@ fn field_container(
                         field_errors.add(tokens, ATTR_BACKTRACE_FALSE);
                     }
                 }
+                Att::Implicit(tokens, v) => {
+                    if v {
+                        implicit_attrs.add((), tokens);
+                    }
+                }
                 Att::Visibility(tokens, ..) => field_errors.add(tokens, ATTR_VISIBILITY),
                 Att::Display(tokens, ..) => field_errors.add(tokens, ATTR_DISPLAY),
                 Att::Context(tokens, ..) => field_errors.add(tokens, ATTR_CONTEXT),
@@ -755,6 +770,9 @@ fn field_container(
         let (backtrace_attr, errs) = backtrace_attrs.finish_with_location();
         errors.extend(errs);
 
+        let (implicit_attr, errs) = implicit_attrs.finish();
+        errors.extend(errs);
+
         let source_attr = source_attr.or_else(|| {
             if field.name == "source" && !source_opt_out {
                 Some((None, syn_field.clone().into_token_stream()))
@@ -771,6 +789,8 @@ fn field_container(
             }
         });
 
+        let implicit_attr = implicit_attr.is_some();
+
         if let Some((maybe_transformation, location)) = source_attr {
             let Field { name, ty, .. } = field;
             let transformation = maybe_transformation
@@ -789,6 +809,8 @@ fn field_container(
             );
         } else if let Some((_, location)) = backtrace_attr {
             backtrace_fields.add(field, location);
+        } else if implicit_attr {
+            implicit_fields.push(field);
         } else {
             user_fields.push(field);
         }
@@ -906,6 +928,7 @@ fn field_container(
     Ok(FieldContainer {
         name,
         backtrace_field: backtrace.map(|(val, _tts)| val),
+        implicit_fields,
         selector_kind,
         display_format,
         doc_comment,
@@ -1010,6 +1033,7 @@ fn parse_snafu_tuple_struct(
                 }
             }
             Att::Backtrace(tokens, ..) => struct_errors.add(tokens, ATTR_BACKTRACE),
+            Att::Implicit(tokens, ..) => struct_errors.add(tokens, ATTR_IMPLICIT),
             Att::Context(tokens, ..) => struct_errors.add(tokens, ATTR_CONTEXT),
             Att::Whatever(tokens) => struct_errors.add(tokens, ATTR_CONTEXT),
             Att::CrateRoot(tokens, root) => crate_roots.add(root, tokens),
@@ -1086,6 +1110,7 @@ enum SnafuAttribute {
     CrateRoot(proc_macro2::TokenStream, UserInput),
     Display(proc_macro2::TokenStream, UserInput),
     DocComment(proc_macro2::TokenStream, String),
+    Implicit(proc_macro2::TokenStream, bool),
     Source(proc_macro2::TokenStream, Vec<Source>),
     Visibility(proc_macro2::TokenStream, UserInput),
     Whatever(proc_macro2::TokenStream),
@@ -1275,6 +1300,7 @@ impl<'a> quote::ToTokens for ContextSelector<'a> {
 
         let context_selector = ContextSelector {
             backtrace_field: self.1.backtrace_field.as_ref(),
+            implicit_fields: &self.1.implicit_fields,
             crate_root: &self.0.crate_root,
             error_constructor_name: &quote! { #enum_name::#variant_name },
             original_generics_without_defaults: &self.0.provided_generics_without_defaults(),
@@ -1306,6 +1332,7 @@ impl<'a> quote::ToTokens for DisplayImpl<'a> {
             .map(|variant| {
                 let FieldContainer {
                     backtrace_field,
+                    implicit_fields,
                     display_format,
                     doc_comment,
                     name: variant_name,
@@ -1315,6 +1342,7 @@ impl<'a> quote::ToTokens for DisplayImpl<'a> {
 
                 let arm = DisplayMatchArm {
                     backtrace_field: backtrace_field.as_ref(),
+                    implicit_fields: &implicit_fields,
                     default_name: &variant_name,
                     display_format: display_format.as_ref().map(|f| &**f),
                     doc_comment,
@@ -1434,6 +1462,7 @@ impl NamedStructInfo {
                     name,
                     selector_kind,
                     backtrace_field,
+                    implicit_fields,
                     display_format,
                     doc_comment,
                     visibility,
@@ -1489,6 +1518,7 @@ impl NamedStructInfo {
 
         let arm = DisplayMatchArm {
             backtrace_field: backtrace_field.as_ref(),
+            implicit_fields: &implicit_fields,
             default_name: &name,
             display_format: display_format.as_ref().map(|f| &**f),
             doc_comment: &doc_comment,
@@ -1510,6 +1540,7 @@ impl NamedStructInfo {
 
         let context_selector = ContextSelector {
             backtrace_field: backtrace_field.as_ref(),
+            implicit_fields: implicit_fields,
             crate_root: &crate_root,
             error_constructor_name: &name,
             original_generics_without_defaults: &original_generics,